The present invention is related to the field of cold cathode, crossed-field discharge switches for high current, high voltage applications.
The present invention is an improvement to the cold cathode, grid-controlled, crossed-field switch which is described in U.S. Pat. No. 4,247,084, "Cold Cathode Discharge Device with Grid Control," assigned to the assignee of the present application. This issued patent is incorporated into this application by this reference.
Generally, the device described in the above-referenced patent comprises a cold cathode, grid-controlled, crossed-field switch which can be repetitively operated in the presence of a fixed magnetic field.
While U.S. Pat. No. 4,247,084 is directed to rapid closing and current control features of the switch, it does not explicitly describe modulator operation or current interruption capability through convenient control grid potential manipulation as may accomplished with hard-vacuum thermionic cathode switches (hard tubes). The patent does indicate (in the abstract and column 4, lines 30-32) that the anode current may be controlled linearly with the control grid. However, it further states (Column 4, lines 36-40) that once the control grid is immersed in the plasma, that grid control may be lost, and that the switch may recover to its nonconducting state (interrupting) by stopping the supply of current to the anode and control grid rather than by simply driving the control grid to negative potentials.
U.S. Pat. No. 4,247,804 references several background patents for cross-field switches: U.S. Pat. Nos. 3,638,061; 3,641,384; 3,604,977; 3,558,960; 3,678,289; 3,769,537; 3,749,978 and 4,034,260.
Another type of switch device commonly employed in medium and high power switch applications is the thyratron. In general the thyratron comprises an anode, a control grid and a thermionic cathode, in an envelope filled with a gas at a relatively high pressure. The tube remains in a non-conducting state with a positive voltage on the anode, provided a potential equal to (or more negative than) the cathode potential is applied to the control grid. During conduction, a sheath of ions around the grid prevents voltage applied to the grid from penetrating to the main discharge body; as a result, grid control is lost. The thyratron may be returned to its non-conducting state only when the anode current is commutated to zero for a recovery time sufficient to allow the charge density to decay sufficiently to allow grid control to be achieved.
A thyratron, then, is a switch which is turned on by positive grid voltage but which may be turned off only by commutation of the anode current. Thyratron operation is described, for example, in the reference "Hydrogen Thyratrons," issued by the GEC Electron Tube Company Limited Company, United Kingdom, 1972.
A modified thyratron device, known as the tacitron, is described in "The Tacitron, A Low Noise Thyratron Capable of Current Interruption by Grid Action," E. O. Johnson, J. Olmstead and W. M. Webster, Proceeding of the I.R.E., September, 1954. The tacitron device described in the reference is understood to be directed to a tube design adapted for operation in a discharge mode wherein ion generation occurs solely in the control-grid-to-anode region. This discharge mode is is said to allow positive ion sheaths from a negative grid to span the grid holes and choke off tube current. The mode is achieved by selection of the overall tube geometry and characteristics, including the size of the grid openings, the gas and its pressure. The tacitron device described in this paper, however, is believed to be adapted to interrupt only relatively small anode currents.
Reference has appeared in literature published in the USSR to tacitron devices said to be adapted to high-power applications. Two such papers are "Powerful Tacitrons and Some of Their Characteristics in a Nanosecond Range," V. D. Dvornikov, S. T. Latushkin, V. A. Krestov, L. M. Tikhomirov, and L. P. Yudin, Pribory i Tekhnika Eksperimenta, July and August 1972, No. 4, 108-110, and "High-Power Tacitron-Based Pulsed Generator," A. S. Aref'ev, V. F. Gnido, and B. D. Maloletkov, Pribory i Tekhnika Eksperimenta, Vol. 2, pp. 117-118, January-February, 1981.
Both the thyratron and tacitron are hot cathode devices which require a continuous high power source to keep the cathode hot. Both devices have an anode and have a control grid. The tacitron employs small grid apertures and relatively low gas pressure (e.g., 0.05 to 0.3 Torr) to provide a current interrupting capability.
It is, therefore, an object of the present invention to provide a cold cathode switch system adapted for modulator operation and switch opening capabilities.
It is another object of the present invention to provide a switch which can be repetitively opened and closed in high current, high voltage applications.
A further object of the present invention is to provide a switch for high voltage, high current applications which can be modulated on and off by a low voltage control.
Still another object of the invention is to provide a cold cathode, crossed-field discharge switch system adapted for control by control grid potential manipulation.